Fix Glance app widget background corner radius

[vitusortner]

The system corner radius (system_app_widget_background_radius) was previously never applied because of a missing return.

Also, the docs of the cornerRadius modifier mention that it only works on SDK 31 and above, which allows us to simplify the corner radius modifiers a bit.

Related PR

Before	After

[shamalip]

The Scaffold component in Glance provides the appropriate background color and radius for rectangular widgets. Example: https://github.com/android/platform-samples/pull/226/commits/bc9a1628fa1a42b77f6c912be0db212131678272#diff-d34a42e99cde7c836c2e038b5ddf4c6550a711ace80438cf04da8434691263be